Abstract

Abandoned waterfalls and dramatic knickpoints line Paulina Creek showing evidence for catastrophic prehistoric floods of unknown origin. In addition to erosional features, there is a large flood deposit (15km2 ) that formed just upstream of the confluence of Paulina Creek and the Little Deschutes River. Paulina Creek originates in Paulina Lake (5.5km2 ) which lies within the western part of the caldera of Newberry Volcano. Newberry Volcano is a Holocene shield-type volcano composed of dominantly basaltic flow, with some flows ranging to rhyolitic composition. Ash flows, cinder cones and pyroclastic deposits also compose the flanks of the volcano. Origin of the floods could range from Paulina Lake water displacement triggered by landslides and/or volcanic eruptions, to a catastrophic collapse of the outlet associated with the upstream migration of a knickpoint. Flood waters exaggerated knickpoints throughout the canyon, originally a result of the stratigraphy of the flanks, and created waterfalls up to 40m high. The Paulina Creek valley is underfit and discharge is uniformly low, flowing at base flow with no high flow events. Under these current conditions the landscape observed could not have formed. Chitwood and Jensen, in their paper on the prehistoric floods, propose a flood discharge of 110-280cms at peak flow, drastically different from normal peak flow (1.5cms). There was no significant change in discharge moving downstream (0.34cms–0.33cms) despite a tripling of watershed size, implying that groundwater influence is low and the majority of the water comes from Paulina Lake. Paulina Lake has high concentrations of arsenic, just above EPA drinking water limits in the water column and much more concentrated in the sediment (0.014ppm and 250ppm respectively). Amount of arsenic in the sediment is dependent on proximity to the lake; Paulina Creek sediments contain more arsenic than the flood plain (6ppm verses 1ppm). Paulina Creek has a cemented bed, which suggests little modern erosion as there is not enough energy to erode the cement. Arsenic from Paulina Lake was characterized for use as a tracer in flood deposits and if such a volcanic event led to displacement of water and sediment from the lake, flood deposits would contain elevated arsenic concentrations. The most reasonable hypothesis for flood origin is a displacement of water triggered by a volcanic event or landslide. Rapid movement along a fault or an avalanche of snow and/or rock displaced water causing failure of the outlet sill. This allowed a large amount of Paulina Lake water to flood Paulina Creek creating the features seen today.